In recent years, replacement of lighting sources from incandescent lamps or fluorescent lamps to energy saving and long life lighting sources such as light-emitting diodes (LED) in luminaires is in progress. Also, for example, new lighting sources such as EL (Electro-Luminescence) or Organic light-emitting diode (OLED) are also developed. Since light outputs from such lighting sources depend on current values flowing therethrough, a power circuit configured to supply a constant current is required when lighting the luminaire. When dimming light, a current to be supplied is controlled. A two-wire dimmer is configured to control the phase which turns triac ON, and is in widespread use as a dimmer of the incandescent lamp. Therefore, the lighting source such as the LED is preferably dimmed by the dimmer. A switching power source such as a DC-DC converter is known as a power source having high efficiency and being suitable for electric power saving and downsizing.
However, the dimmer is configured to be connected in series with a filament of an incandescent lamp acting as a load to allow a current equal to or larger than a holding current required for the dimmer at all phases to flow, thereby operating. Therefore, when the switching power source is connected, a period during which a load impedance is changed and the holding current does not flow occurs, and hence a malfunction may be resulted.